1. Field of the Invention
The present invention relates to methods for the stimulation of T cells having a desired antigen specificity.
2. Description of the Prior Art
The different sub-populations of T cells play an important role on the one hand in the regulation of immunological processes and on the other hand as the effector cells of cytotoxicity in the defense of infectious diseases and malignant growth of new tumors as well as in the development of autoimmune diseases. Therefore, a modulation of T cell activity is of central interest in the context of the treatment of numerous diseases. According to the invention, "stimulation" refers not only to an enhancement of a preexisting T cell-mediated immune response but also to its induction to achieve a defense against pathogens or tumor cells; furthermore, in the case of autoimmune diseases and allergies it is desired to suppress the undesired immune response.
In the past it has been tried in several ways to induce a T cell reaction. On the one hand, the native antigen [2] or antigenic peptides [3, 4] have been employed in vivo and in vitro to induce a T cell response, on the other hand the gene encoding the respective antigen has been introduced in suitable cells by means of gene tranfer and the cells thus modified have been employed as antigen-presenting cells (APCs). Because of their superior immunostimulatory properties, particularly dendritic cells (DCs) have been used for this purpose [5].
These methods bear several advantages and disadvantages. Thus, the use of native antigens is not suitable for an induction of cytotoxic T cells (CTLs). CTLs recognize the relevant antigenic peptide i.e. in combination with class I molecules of the major histocompatibility complex (MHC). However, generally the peptides presented in combination with class I MHC molecules are derived from intra-cytoplasmic antigen processing. Since exogenously added antigens do not enter the class I way of antigen processing it is generally impossible to induce CTLs by immunization with native antigens.
Peptides, in contrast, have the advantage of being able to externally bind to MHC molecules without having to enter the cytoplasm of APCs. However, a prerequisite for the use of peptides for T cell generation or T cell stimulation, respectively, is that the antigenic determinants (epitopes) of the individual antigens must be known. Since MHC molecules can only bind to peptides having conserved structural features but since these consensus sequences are determined by the MHC molecule and are different for each allele, it would be necessary to re-determine the suitable peptides for each MHC haplotype/antigen combination. This not only takes up a great deal of time and effort but, in addition, the technology required is not always available. Moreover, at least the use of peptides in vivo is questionable because in addition to induction of a specific T cell immunity it may also cause an induction of a specific tolerance [6].
The use of antigen-presenting DCs in the immunization is limited by difficulties in isolation, culture and gene transfer into these cells and, furthermore, by strong limitations in the number of DCs available for many diseases.
Besides DCs, the activated B cells are the most potent APCs known. Interestingly, the immortalization of B cells with Epstein-Barr virus results in cells (lymphoblastoid cell lines, LCL) representing the phenotype of activated B cells [7]. These cells are excellent APCs, have a nearly unlimited potential to proliferate and, thus, are also used as such for numerous immunological experiments in the human system for which a regular access to other APCs is not necessary and/or not justified for ethical reasons. Basically, LCLs should be suited as APCs for the induction of any desired T cell response. However, this is impeded by the fact that such cells also induce a strong EBV-specific T cell-mediated immune response interfering with the desired immune reaction [8, 9, 10]. Since the frequency of EBV in the population is extremely high and the frequency of EBV-specific T cells in persons with latent EBV infection is one of the highest T cell frequencies known (only comparable to the frequency of allospecific T cells) EBV-immortalized autologous B cells are well suited for re-stimulation but are unsuitable for a primary stimulation of T cells which do not recognize EBV antigens.